1. Field of the Invention
This invention relates to servomechanisms and particularly to such mechanisms for controlling the position of a payload.
2. Discussion of Prior Art
One position control mechanism known to the inventor operates using feedback from an appropriate position measurement device such as a linear transducer, resolver or optical encoder to control a D.C. motor. Often, the DC motor is configured as a torquer by employing an inner feedback loop to control the armature or field current. One example of such a known system is shown in FIG. 1.
In FIG. 1, a torquer 1 comprising a current controller 2 and DC motor 3, is fed with a torque demand signal T.sub.d. The torque demand is produced by an angle controller 4 which acts on an angular error signal `E`, formed from the difference between demanded and achieved angular position .theta.d and .theta.a respectively. The achieved angular position .theta.a is monitored by an angular measurement device 5. The torquer 1, operating within limits resulting from the supply voltage within the current controller 2, applies torque to a load 6 in order to drive the error signal E to zero. The load 6 could be a gimbal mechanism, for example.
The inventor has found that the angular measurement devices in common use give false readings when driven above a critical rate. When this rate is exceeded, not only does the known system have the disadvantage of providing an inaccurate value of angular position .theta.a, but also, it has been observed that the effect leads to instability in the feedback loop.